A conventional, single-frequency clock generator is shown in FIG. 1A. In this clock generator, a crystal X.sub.L1 which is capable of oscillation generally utilizes quartz as a piezoelectric element, i.e., an element in which strain generates a voltage and vice-versa. Hence, an electric field applied to the crystal generates acoustic waves in the crystal which, in turn, provide a voltage at the surface of the crystal. This crystal, with appropriate contacts formed thereon (typically by plating), forms an RLC circuit which can be pretuned to a particular frequency.
As shown in FIG. 1A, crystal X.sub.L1 is connected in parallel with resistor R.sub.f and inverter A.sub.1, and in series with capacitors C.sub.i and C.sub.o. Inverter A.sub.1 provides the voltage source that initiates oscillation. Assuming node 34 has a low voltage, inverter A.sub.1 provides a high voltage on node 31. This high voltage charges capacitor C.sub.o and causes a current to flow through resistor R.sub.f to node 34. The magnitude of this current depends upon the value of resistor R.sub.f and the voltage difference between node 31 and node 34. The voltage on node 34 increases as capacitor C.sub.i stores charge. When the voltage on node 34 reaches a certain level, the output voltage from inverter A.sub.1 on node 31 changes from high to low, causing capacitor C.sub.o to discharge. Quartz crystal X.sub.L1, after reaching its resonant frequency, stabilizes the oscillator generator circuit 32 at that frequency. The charging/discharging of capacitors C.sub.o and C.sub.i transforms the clear high or low voltage at node 31 provided by inverter A.sub.1 into intermediate voltages. Therefore, to ensure that the output line 33 provides a clear high or low voltage, inverter A.sub.2 is included in the oscillator generator circuit 32.
During high frequency oscillation, i.e. 20 MHz or higher, the above device is limited by the physical characteristics of the inverters A1, A2 and the quartz crystal X.sub.L1. Specifically, if the gain bandwidth product of the inverters A1, A2 is insufficiently large, inverters A1, A2 are unable to generate the needed oscillation frequencies. Moreover, if the stability of quartz crystal X.sub.L1 is insufficient (for example, an inadequate gain of inverter A.sub.1 may cause the phase of oscillation of quartz crystal X.sub.L1 to fail to reach a full 360.degree., or an inadequate crystal polish may induce a very high resonant resistance) or the cut-off of quartz crystal X.sub.L1 is inadequate (i.e wherein the bordering frequency signal is disabled), resonant resistance will be very high, i.e. higher than 50 ohms, thereby making it difficult to initiate oscillation.
Additionally, at these high frequencies, the parasitic capacitance and the bypass capacitance of the clock generator circuit have an even greater negative effect on the stability of the circuit than do inverters A1, A2 and quartz crystal X.sub.L1, especially if a plurality of clock generators placed together are oscillating simultaneously. Therefore, it is important that resistor R.sub.f, capacitors C.sub.o and C.sub.i, and quartz crystal X.sub.L1 are properly configured in the design of the device.
During mass production, the characteristics of the materials used to produce each new batch of devices are different. Thus, values for resistor R.sub.f and capacitors C.sub.o and C.sub.i must frequently be adjusted for each batch of devices to ensure a reliable clock generator circuit. This continual adjustment results in a needless waste of manpower, materials, and time. At the same time, this adjustment makes quality assurance standards difficult to maintain. Frequently, failure to meet quality assurance standards results in irreparable harm to a product's image. Another quality problem arises because resistors and capacitors are frequently susceptible to changes in weather, particularly, temperature and to dust, thereby changing the oscillation characteristics and the stability of the entire clock generator circuit. Preventing the above problems poses many serious difficulties in designing and manufacturing of the circuit.
Another prior art single-frequency clock generator is shown in FIG. 1B. Note that clock generator 35, similar to clock generator 32 illustrated in FIG. 1A, has limitations during high frequency oscillation. Specifically, clock generator 35 is limited by the physical characteristics of the transistor Q.sub.1, and the quartz crystal X.sub.L1. Thus, clock generator 35 has many of the same disadvantages inherent in clock generator 32 described above in detail.
Many electronics products on the market, such as video graphics arrays (VGAs) require different clock frequencies for each of their different functions. Thus, VGAs often require up to four clock generators to provide the necessary frequencies. FIG. 1C shows a schematic circuit diagram of the clock generator shown in FIG. 1A in a VGA application. Clocks generators 32A, 32B, 32C and 32D each provide a different frequency via lines 36 to VGA chip 37. In one example, crystals X.sub.L1A, X.sub.L1B, X.sub.L1C and X.sub.L1D provide a frequency of 50.35 MHz, 28.322 MHz, 44.9 MHz, and 36.0 MHz, respectively. In the above example, resistors R.sub.fA and R.sub.fC provide a resistance of 2K Ohm while resistors R.sub.fb and R.sub.fd provide a resistance of 5.1k Ohm and 3.6k Ohms, respectively. Additionally, capacitors C.sub.iA, C.sub.oA, C.sub.oB, C.sub.iC, C.sub.oC, and C.sub.oD have a capacitance of 10 picofarads, whereas capacitors C.sub.iB and C.sub.iD have a capacitance of 30 picofarads. As shown in FIG. 1C, VGA chip 37 is coupled to 24 components. A few illustrative examples of VGAs requiring multiple clock generators are Model W86875 VGA chip manufactured by Winbond Electronics Corp. and Model WD 90C11 VGA chip manufactured by Western Digital.
In a multiple clock generator configuration, the interference between the clock generators caused by the coupling effect from different frequencies, the distribution of parasitic capacitance that causes distortion, and the accumulated noise have a significant effect on the entire clock generator circuit and make quality control during production very difficult. Therefore a need arises for a high frequency clock generator which (1) allows for selection between various frequencies, (2) permits use of different frequencies at the same time, (3) reduces the interference between a plurality of clock oscillation circuits, (4) minimizes external factors which may degrade performance, and (5) eliminates problems in mass production.